The growth and retraction of dendritic spines with synapse formation and elimination is thought to underlie experience-dependent changes in brain circuitry during development and in the adult brain, and also may play a role in neurodevelopmental disorders. While much focus has been given to spine growth and associated synapse formation as a key step in the development of brain circuits, the mechanisms and functional implications of spine retraction have been neglected. During late postnatal development, after initial connectivity has been established, dendritic spine densities decrease and half of all synapses are lost in some regions of the cortex. This period coincides with a period of intense learning, suggesting that spine retraction and synapse elimination may have an integral role in learning and memory. The primary goal of the proposed studies is to determine the mechanisms that govern the retraction of dendritic spines and the disassembly of spine synapses during brain development, plasticity, and disease. We have three specific aims. First, we will test our hypothesis that spine retraction is induced by activity patterns that lead to synaptic weakening. Second, we will determine whether spine retraction is preceded, and possibly even initiated, by synapse disassembly. Finally, we will examine whether turnover rates of postsynaptic proteins can influence spine stability. To achieve these goals, we will use focal photolysis of caged glutamate to stimulate individual spines, combined with electrophysiology to measure consequences on spine synapse function, and dual-color time-lapse imaging to monitor the dynamics of postsynaptic proteins during spine retraction. The combined use of two-photon imaging techniques and electrophysiology of single synapses provides a novel way to identify the mechanisms that govern the retraction and disassembly of spine synapses. Results from our experiments will fill major gaps in our current understanding of neural circuit refinement during experience-dependent plasticity. Ultimately, basic knowledge of the mechanisms of spine synapse elimination has strong potential to facilitate the development of therapeutics for neurological diseases.